Stamp the plan doc with shipped-by-phase status; add the multi-user Key Patterns
entry + State Files + case-spaces note to CLAUDE.md; add a multi-user section to
the security architecture (threat model: workspace separation, not a security
boundary) and a README opt-in section; add a minor changeset.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
